Greenhill executive Jeff Buckalew and his family were killed when their single-engine plane crashed in New Jersey. Photo:

WASHINGTON — Investigators have yet to reach a conclusion on what caused a small plane to crash on a New Jersey highway last week, killing all five people on board, according to a preliminary report released Wednesday by the National Transportation Safety Board (NTSB).

The plane crashed onto a wooded median on Interstate 287 in Morristown, N.J., on Dec. 20 after “an in-flight loss of aircraft control,” the report said. But what caused the loss of control remains unknown.

Last week, NTSB investigators said air traffic control had maintained communication with the pilot until shortly before the crash. The pilot, Jeffrey Buckalew, had reported some icing and requested a climb. The plane reached a maximum altitude of 17,900 feet before beginning to descend, the NTSB said.

The aircraft subsequently lost radar and radio contact.

The NTSB report said investigators moved the wreckage of the single-engine Socata TBM 700 into a storage facility, “where a detailed examination will be performed.” That investigation could take up to a year.

Killed in the crash were Buckalew, 45, an investment banker from New York, his wife Corrine, their two children and Buckalew’s colleague Rakesh Chawla, 36. The five victims had been on their way to Atlanta.

There were conflicting witness reports about the crash, with some saying the plane was intact until it hit the ground and others saying it broke apart in the air.

The NTSB said the plane’s wreckage was strewn over a quarter-mile area, with parts of the aircraft landing in a wooded area on the east side of the highway’s northbound lanes, the fuselage crashing on the highway median and other sections landing in a nearby residential area.